B. burgdorferi naturally contains a large number of extrachomosomal DNAs (plasmids), many of which carry the genes for important Lyme disease antigens. Other genes that may be important for bacterial pathogenesis are also located on these small DNA elements. Furthermore, there is good evidence that there has been exchange of plasmids between B. burgdorferi cells, a common method among bacteria to increase the diversity of antigenic proteins. We have undertaken the study of several of these plasmids with the goal of understanding the roles of these DNAs and their encoded proteins in the life cycle of B. burgdorferi and the transmission of Lyme disease.